Floating marine structure and method for controlling temperature thereof

ABSTRACT

Disclosed herein are a floating structure and a temperature control method of a floating structure. In more detail, the floating structure includes a cofferdam provided between a plurality of LNG storage tanks installed in at least one row in a length direction of a hull, in which a temperature of the cofferdam is controlled to be temperature below 0° C. to decrease a boil-off rate (BOR) generated by transferring heat from the cofferdam into the plurality of LNG storage tanks.

TECHNICAL FIELD

The present invention relates to a floating structure and a temperature control method of a floating structure, and more particularly, to a floating structure and a temperature control method of a floating structure capable of decreasing a boil-off rate (BOR) by decreasing a heat transfer between a cofferdam and an LNG stored in an LNG storage tank.

BACKGROUND ART

Generally, natural gas is transported in a gas state through a ground or maritime gas pipe and is stored in an LNG carrier in a liquefied natural gas (hereinafter, referred to as ‘LNG’) state to be transported to remote consumption sites.

The LNG obtained by cooling the natural gas at a very low temperature (approximately, −163° C.) has a volume decreased to approximately 1/600 compared to that of the natural gas in a gas state. Therefore, the LNG is very suitable for long distance transportation through the sea.

The LNG may be transported through the sea while being carried in the LNG carrier to be loaded and unloaded at ground consumption sites or may be transported through the sea while being carried in an LNG regasification vessel (LNG RV) to reach ground consumption sites. After that, the LNG may be regasified to be loaded and unloaded in a natural gas state. For this purpose, the LNG carrier and the LNG RV include an LNG storage tank (referred to as ‘cargo hold’) that may bear very low temperature LNG).

Further, a demand for floating structures such as LNG floating, production, storage and offloading (LNG FPSO) and LNG floating storage and regasification unit (LNG FSRU) is gradually increased. The floating structure also includes the LNG storage tank that is installed in the LNG carrier or the LNG RV.

Here, the LNG FPSO is a floating structure that directly liquefies the produced natural gas on the sea and stores the liquefied natural gas in the storage tank and if necessary, is used to transship the LNG stored in the storage tank to the LNG carrier.

The LNG FSRU is a floating structure that stores the LNG loaded and unloaded from the LNG carrier in the storage tank on the sea remote from the ground and then gasifies the LNG if necessary and supplies the gasified LNG to ground consumption sites.

The LNG storage tank is classified into an independent tank type and a membrane type depending on whether a heat insulating material for storing LNG in a very low temperature state is directly applied to a load of freights. Here, the independent tank type storage tank is classified into an MOSS type and an IHI-SPB type and the membrane type storage tank is classified into a GT NO 96 type and a TGZ Mark III type.

Among the existing LNG storage tanks, the GT NO 96 type that is the membrane type has a structure in which a primary barrier and a secondary barrier made of an invar steel (36% Ni) having a thickness of 0.5 to 0.7 mm are installed on an inner surface of a hull in two layers, in which the primary barrier is positioned at an LNG side and the secondary barrier is positioned at the inner surface of the hull to enclose the LNG doubly.

Further, a space between the primary barrier and the secondary barrier is provided with a primary heat insulating wall and a space between the secondary barrier and an internal hull is provided with a secondary heat insulating wall, in which the primary heat insulating wall and the secondary heat insulating wall minimize that heat outside the LNG storage tank is transferred to the LNG.

Meanwhile, since the LNG stored in the LNG storage tank is stored at approximately −163° C. that is a gasification temperature at a normal pressure, if heat is transferred to the LNG, the LNG is gasified and thus boil off gas (hereinafter, referred to as ‘BOG’) occurs.

Further, in the case of the membrane type LNG storage tank, if the cold LNG storage tanks are continuously installed, a temperature of steel between the cold LNG storage tanks is suddenly decreased, and therefore a brittle fracture may occur.

To prevent the brittle fracture, a space called the cofferdam is installed between the LNG storage tanks and thus the LNG storage tank is prevented from being damaged due to the low temperature LNG.

However, even though the cofferdam is installed, the temperature of steel of a cofferdam bulk head member contacting the LNG cargo hold may be dropped to −60° C. or less due to the very low temperature LNG. If the general steel is exposed to −60° C., the cofferdam is damaged due to low temperature brittleness.

As a plan to overcome the above problem, the cofferdam may be made of a very low temperature material such as stainless steel and aluminum. However, if the very low temperature material is used, ship prices may suddenly rise.

Therefore, when the cofferdam and the LNG storage tank are installed, the temperature of the cofferdam is controlled to be 5° C. and the cofferdam bulk head is made of relatively cheap steel that can bear the room temperature.

In the case of the existing LNG carrier, when the temperature of the cofferdam is equal to or less than 5° C., a heating system is operated and thus the cofferdam is always maintained at 5° C. or more. For this purpose, the existing LNG carrier includes a glycol heating system or an electrical heating system.

Therefore, the existing LNG carrier is designed/sailed so that the temperature of the cofferdam may be equal to or more than at least 5° C. all the time and a BOR is also generated under the temperature condition.

DISCLOSURE Technical Problem

In the current LNG carrier market, the LNG carrier is sensitive to numerical values of the BOR at a contract stage. As an actual example, typically, the BOR of 0.15% is a contract condition, but in recent years, the BOR of 0.125%, 0.10%, 0.095%, or the like as the contract condition may be suggested.

However, in the current membrane type tank, the heat insulating wall is installed in the cargo hold Since the heat insulating wall of the LNG cargo hold needs to bear and transfer the load applied from the LNG freight to the cargo hold while having heat insulating performance, the existing heat insulating wall of the LNG cargo hold is changed to increase the heat insulating performance, a lot of research and design changes may be involved and costs may rise.

In fact, even though there is the insulating wall of the LNG cargo hold satisfying the BOR of about 0.13%, if the BOR requirement of a ship-owner is 0.125%, to decrease the BOR as much as about 4%, a lot of research, time, and costs are involved.

Further, even though there is the insulating wall of the LNG cargo hold securing the BOR of 0.103%, if the ship-owner proposes the BOR of 0.10%, shipyards may not apply the LNG cargo hold and therefore may not accept an order for the LNG carrier. Today's LNG carrier market is in a situation that a shipyard achieving even 1% decrease in the BOR may dominate an order competition for the LNG carrier over other shipyards.

Meanwhile, the existing technology development for decreasing the BOR focuses on improvement in the performance of the insulating wall of the LNG cargo hold. Since today's market demands even 1% decrease in the BOR, a method mostly discussed at present is to increase a thickness of the LNG cargo hold.

However, a volume of the cargo hold that may store LNG is decreased in proportion to the increase in the thickness of the LNG cargo hold. On the other hand, to prevent the volume of the cargo hold from decreasing, a size of a ship is increased.

Further, if the thickness of the cargo hold is increased, the cargo hold is weaker structurally. Therefore, researches for reinforcing the cargo hold have to be conducted.

An aspect of the present invention is to provide a floating structure capable of decreasing a BOR with a low cost by lowering a control temperature of a cofferdam and designing and manufacturing a cofferdam bulk head on a steel grade corresponding thereto.

Another aspect of the present invention is to provide a floating structure and a temperature control method of a floating structure capable of controlling a control temperature of a cofferdam depending on sailing conditions controlled to be temperature below 0° C. and a type of work while decreasing a BOR with a low cost by lowering the control temperature of the cofferdam.

Still another aspect of the present invention is to provide a floating structure capable of easily confirming a cold spot, or the like of a cofferdam while decreasing a BOR with a low cost by lowering a control temperature of the cofferdam.

Still yet another aspect of the present invention is to provide a floating structure capable of satisfying required structural strength while decreasing a BOR by decreasing a heat transfer through a bulk head.

Technical Solution

According to an embodiment of the present invention, there is provided a floating structure, including: a cofferdam provided between a plurality of LNG storage tanks installed in at least one row in a length direction of a hull, in which a temperature of the cofferdam is controlled to be temperature below 0° C. to decrease a boil-off rate (BOR) generated by transferring heat from the cofferdam into the plurality of LNG storage tanks.

The cofferdam may include: a pair of bulk heads spaced apart from each other between the plurality of LNG storage tanks; and a space portion provided by the pair of bulk heads and an inner wall of the hull, and the cofferdam controls the pair of bulk heads to be temperature below 0° C.

The pair of bulk heads may be made of at least one material of B, D, E, AH, DH, and EH that are a steel grade defined in IGC.

The pair of bulk heads may be made of low temperature steel LT applied at −30° C. or less.

The pair of bulk heads may be controlled to be −30° C. to −20° C. and may be made of the E or the EH that is the steel grade defined in the IGC.

The floating structure may further include: a gas supplier supplying gas into the cofferdam to prevent an inside of the cofferdam from being damaged due to freezing of moisture in air.

The gas supplier may include: a supply pipe provided in the hull to supply the gas into the cofferdam; a discharge pipe provided in the cofferdam to discharge the gas in the cofferdam to an outside of the cofferdam; and valves provided in the supply pipe and the discharge pipe.

The gas may include at least one of dry air, inert gas, or N₂ gas.

The floating structure may further include: a heater provided in the cofferdam to heat the cofferdam, in which the cofferdam may be controlled to be temperature below 0° C. to decrease the boil-off rate (BOR) generated by transferring heat from the cofferdam into the plurality of LNG storage tanks and may be heated by the heater to change the temperature below 0° C. to a specific temperature in addition to temperature above 0° C.

When a bulk head of the cofferdam is made of a material bearing a temperature from −30° C. to 0° C., the temperature of the cofferdam may be changed in a range from −30° C. to 70° C.

When a bulk head of the cofferdam is made of low temperature steel bearing up to −55° C., the temperature of the cofferdam may be changed in a range from −55° C. to 70° C.

When fuel consumption of the floating structure is increased, the temperature of the cofferdam may be increased to increase the generation of the boil-off gas (BOG) and thus the BOG may be used as fuel and when the fuel consumption of the floating structure is decreased, the temperature of the cofferdam may be lowered to decrease the generation of the BOG.

The heater may heat the cofferdam to control the temperature of the cofferdam to be a specific temperature in addition to temperature above 0° C. so that a worker may enter the cofferdam.

The temperature below 0° C. of the cofferdam may be changed to the specific temperature in addition to the temperature above 0° C. due to high-temperature dry air supplied into the cofferdam.

When a pressure in the LNG storage tank is larger than a set pressure of the LNG storage tank, a set temperature of the cofferdam may be lowered and when the pressure in the LNG storage tank is lower than the set pressure of the LNG storage tank, the set temperature of the cofferdam may be increased.

The heater may heat at least one of a trunk deck space controlled to be temperature below 0° C. and a side passage way contacting a trunk deck to change the temperature of the trunk deck space and the side passage way to the specific temperature in addition to the temperature above 0° C.

The floating structure may further include: a heat insulating material provided in the cofferdam.

The cofferdam may include a plurality of lateral cofferdams that segment the plurality of LNG storage tanks laterally and the heat insulating material may be provided in a foremost bulk head of a bow of the lateral cofferdam disposed at a foremost side of the bow and a rearmost bulk head of a stern of the lateral cofferdam disposed at a rearmost side of the stern, respectively, among the plurality of lateral cofferdams.

The heat insulating material may include at least one of a heat insulating wall heat-insulating the LNG stored in the plurality of LNG storage tank, a panel type heat insulating material, a foam type heat insulating material, a vacuum heat insulation or particle type heat insulating material, and a non-woven fabrics type heat insulating material.

The floating structure may further include a heat insulating material damage prevention member provided at a bottom part of the cofferdam to prevent the heat insulating material from being damaged.

The floating structure may further include: a gas supplier supplying gas to the cofferdam.

The gas supplier may include: a gas supply pipe provided in the cofferdam to supply the gas supplied through a gas supply line into the cofferdam; a gas discharge pipe provided in the cofferdam to discharge the gas in the cofferdam to an outside of the cofferdam; and shutoff valves provided in the gas supply pipe and the gas discharge pipe.

The gas supplied into the cofferdam may have a dew point temperature from −45° C. to −35° C. and the pair of bulk heads may be controlled to be 1° C. to 10° C. higher than the dew point temperature of the gas.

The temperature of the cofferdam may be maintained at temperature above 0° C. while continuously injecting and venting the gas into the cofferdam and the gas may have temperature above 0° C.

The temperature of the cofferdam may be increased by continuously injecting and discharging the gas into and from the cofferdam to provide an environment that a worker enters the cofferdam.

The gas supplier may supply the gas into at least one of the trunk deck space controlled to be temperature below 0° C. and the side passage way contacting the trunk deck and the dew point temperature of the gas may be lower than the temperature of the trunk deck space and the side passage way.

The gas may include dry air.

The bulk head may not be extended up to an external hull but may be connected only to an internal hull and a strength member connecting between the external hull and the internal hull may be provided not to be continued to the bulk head to decrease the boil-off rate (BOR) generated by transferring heat between the bulk head and the LNG stored in the plurality of LNG storage tanks.

The bulk head may be controlled to be a temperature from −163° C. to −50° C. and may be made of a very low temperature material including aluminum or stainless steel.

The floating structure may further include: a sealing and heat insulating unit provided in the plurality of LNG storage tanks to seal and heat-insulate the LNG, in which the sealing and heat insulating unit may not be provided in the bulk head of a region in which the plurality of LNG storage tanks and the bulk head contact each other.

A space portion may be provided between the bulk heads disposed at a foremost side of a bow and a rearmost side of a stern and the internal hull and may be provided with a heat insulating material.

The heat insulating material may include at least one of the heat insulating wall heat-insulating the LNG stored in the plurality of LNG storage tank, the panel type heat insulating material, the foam type heat insulating material, the vacuum heat insulation or particle type heat insulating material, and the non-woven fabrics type heat insulating material.

The internal hull may be made of a very low temperature material.

The floating structure may be any one selected from an LNG carrier, an LNG FPSO, an LNG RV, and an LNG FSRU.

According to another embodiment of the present invention, there is provided a temperature control method of a floating structure, including: controlling a cofferdam at a specific sub-zero temperature to decrease a BOR; controlling the temperature of the cofferdam to a specific temperature in addition to temperature above 0° C. so that a worker enters the cofferdam controlled to be the sub-zero temperature; and controlling the temperature of the cofferdam to be the specific sub-zero temperature again when the worker gets out of the cofferdam.

The temperature of the cofferdam may be controlled to be a range from −55° C. to 70° C.

The cofferdam provided between the plurality of LNG storage tanks may be controlled to be temperature below 0° C. to decrease the boil-off rate (BOR) generated by transferring heat from the cofferdam into the plurality of LNG storage tanks and may be heated by the heater provided in the hull to change the temperature below 0° C. of the cofferdam to a specific temperature in addition to temperature above 0° C.

The gas may be supplied to the cofferdam provided between the plurality of LNG storage tanks and controlled to be temperature below 0° C. and the dew point temperature of the gas may be lower than the temperature of the bulk head of the cofferdam.

The cofferdam provided between the plurality of LNG storage tanks installed in at least one row in the length direction of the hull may be controlled to be temperature below 0° C. to decrease the boil-off rate generated by transferring heat from the cofferdam into the plurality of LNG storage tanks.

The cofferdam provided between the plurality of LNG storage tanks installed in at least one row in the length direction of the hull may be controlled to be temperature below 0° C. to decrease the boil-off rate generated by transferring heat from the cofferdam into the plurality of LNG storage tanks and the cofferdam may be provided with the heat insulating material.

The cofferdam provided between the plurality of LNG storage tanks installed in at least one row in the length direction of the hull may be provided with the heat insulating material.

The bulk head provided between the plurality of LNG storage tanks may not be extended up to an external hull but may be connected only to an internal hull and the strength member connecting between the external hull and the internal hull may be provided not to be continued to the bulk head to decrease the boil-off rate (BOR) generated by transferring heat from the bulk head into the plurality of LNG storage tanks.

The bulk head partitioning the plurality of LNG storage tanks may be made of the very low temperature material, the pair of bulk heads may be disposed at the foremost side of the bow and the rearmost side of the stern while being spaced apart from each other, and the space portion other than the bulk head contacting the LNG storage tank may be provided with the heat insulating material.

Advantageous Effects

The embodiments of the present invention may control the temperature of the cofferdam to be temperature below 0° C., thereby decreasing the boil-off rate (BOR) generated by transferring heat between the cofferdam and the LNG stored in the plurality of LNG storage tanks.

That is, the embodiments of the present invention do not decrease the BOR by deforming the complex and expensive LNG cargo hold but fundamentally decrease the heat invasion into the LNG cargo hold by lowering temperature around the LNG cargo hold, thereby decreasing the BOR while maintaining the transportation efficiency of the LNG freight.

Further, some of the embodiments of the present invention may perform a control to increase the control temperature when the BOG is less generated to generate more BOG and lower the control temperature when the BOG is more generated to generate less BOG and may control the cofferdam to be temperature above 0° C. when a worker needs to enter the cofferdam to inspect the inside of the cofferdam to allow the worker to enter the cofferdam.

DESCRIPTION OF DRAWINGS

FIG. 1 is a side view schematically illustrating a state in which a cofferdam is installed in a floating structure according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along the line II-II of FIG. 1.

FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 1.

FIG. 4 is a plan cross-sectional view illustrating a state in which the cofferdam is provided between LNG storage tanks disposed in two rows in the floating structure illustrated in FIG. 1.

FIG. 5 is a cross-sectional view taken along the line IV-IV of FIG. 4.

FIG. 6 is a table showing a steel grade defined in IGC.

FIG. 7 is a table showing calculated results of a BOR generated by controlling a temperature of the cofferdam in the first embodiment of the present invention.

FIG. 8 is a diagram schematically illustrating a state in which a heater is provided in the floating structure according to the first embodiment of the present invention.

FIG. 9 is a diagram schematically illustrating a state in which a heat insulating material is provided in a cofferdam in a heat insulating system of a floating structure according to a second embodiment of the present invention.

FIG. 10 is a perspective view schematically illustrating a state in which the heat insulating material is provided in region “A” of FIG. 9.

FIG. 11 is a perspective view schematically illustrating a state in which the heat insulating material is provided in region “B” of FIG. 9.

FIG. 12 is a diagram schematically illustrating a heat insulating material damage prevention member provided to prevent the heat insulating material from being damaged in region “C”.

FIG. 13 is a table illustrating calculated results of a BOR generated by controlling the temperature of the cofferdam using the heat insulating material illustrated in FIG. 9.

FIG. 14 is a diagram schematically illustrating a state in which a bulk head of a cofferdam in a floating structure according to a third embodiment of the present invention is not extended to an external hull but is connected only to an internal hull.

FIG. 15 is a modified example of FIG. 14 in which instead of the bulk head illustrated in FIG. 14, the cofferdam is provided and the heat insulating material is provided in the cofferdam.

FIG. 16 is a table illustrating calculated results of a BOR generated by manufacturing the bulk head illustrated in FIG. 13 using a very low temperature material and controlling the temperature of the cofferdam.

FIG. 17 is a diagram schematically illustrating a gas supplier in a floating structure according to a fourth embodiment of the present invention.

FIG. 18 is a table illustrating calculated results of a BOR generated by controlling a temperature of a cofferdam illustrated in FIG. 17.

FIG. 19 is a diagram schematically illustrating controlling the temperature of the cofferdam depending on a change in pressure of an LNG storage tank in a floating structure according to a fifth embodiment of the present invention.

FIG. 20 is a diagram schematically illustrating a state in which a heat insulating material is provided in a trunk deck space and a side passage way in a heat insulating system of a floating structure according to a sixth embodiment of the present invention.

FIG. 21 is a table showing calculated results of a BOR generated by controlling a temperature of an internal hull contacting the trunk deck space and the side passage way illustrated in FIG. 20.

FIG. 22 is a diagram schematically illustrating a state in which a heat insulating material is provided in a ballast tank in a heat insulating system of a floating structure according to a seventh embodiment of the present invention.

FIG. 23 is a table illustrating calculated results of a BOR generated by controlling a temperature of an internal hull contacting the ballast tank.

DETAILED DESCRIPTION OF MAIN ELEMENTS

1, 200, 300, 400: Floating structure

100, 500, 600: Heat insulating system of floating structure

10: Cofferdam 30: Heater

120: Heat insulating material 220: Strength member

320: Gas supplier

Embodiments

In order to sufficiently understand the present invention, operational advantages of the present invention, and objects accomplished by embodiments of the present invention, the accompanying drawings showing embodiments of the present invention and contents described in the accompanying drawings should be referred.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Like reference numerals proposed in each drawing denote like components.

In the present specification, a floating structure includes a ship and various structures that are used while floating on the sea, including a storage tank for storing LNG and may include an LNG floating, production, storage and offloading (LNG FPSO), an LNG floating storage and regasification unit (LNG FSRU), an LNG carrier, and an LNG regasification vessel (LNG RV).

FIG. 1 is a side view schematically illustrating a state in which a cofferdam is installed in a floating structure according to a first embodiment of the present invention, FIG. 2 is a cross-sectional view taken along the line II-II of FIG. 1, FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 1, FIG. 4 is a plan cross-sectional view illustrating a state in which the cofferdam is provided between LNG storage tanks disposed in two rows in the floating structure illustrated in FIG. 1, FIG. 5 is a cross-sectional view taken along the line IV-IV of FIG. 4, FIG. 6 is a table showing a steel grade defined in IGC, FIG. 7 is a table showing calculated results of a BOR generated by controlling a temperature of the cofferdam in the first embodiment of the present invention, and FIG. 8 is a diagram schematically illustrating a state in which a heater is provided in the floating structure according to the first embodiment of the present invention.

According to the present embodiment, a cofferdam 10 is controlled to be temperature below 0° C. to decrease a boil-off rate (BOR) generated by transferring heat from the cofferdam 10 into a plurality of LNG storage tanks T.

As illustrated in the drawings, a floating structure 1 according to the present embodiment includes the cofferdam 10 provided between the plurality of LNG storage tanks T installed in at least one row in a length direction of a hull and controlled to be temperature below 0° C.

The cofferdam 10 is provided in the plurality of LNG storage tanks T installed in at least one row in the length direction of the hull and as illustrated in FIGS. 1 to 3, may be provided between the plurality of LNG storage tanks T installed in multi rows in the length direction of the hull or as illustrated in FIGS. 4 and 5, provided between the plurality of LNG storage tanks T installed in two rows in a width direction and the length direction of the hull.

Unlike the related art, according to the present embodiment, the cofferdam 10 is controlled to be temperature below 0° C. to decrease the BOR.

In detail, the related art always maintains a temperature of the cofferdam at 5° C. or more. The reason is that when the temperature of the cofferdam is controlled to be lower than 5° C., a temperature of the bulk head 11 of the cofferdam using steel grade A defined in IGC is lowered below 0° C. and the bulk head 11 may suffer from brittle fracture.

When the temperature of the cofferdam is maintained at 5° C. or more as described above, the BOR is generated by heat transfer due to a temperature difference between the cofferdam and the LNG stored in the LNG storage tank T. For example, for an actual ship built in a shipyard, as illustrated in a table of FIG. 6, a BOR of 0.1282 is calculated.

However, according to the present embodiment, when the temperature of the cofferdam 10 is controlled to be temperature below 0° C., the temperature difference between the LNG and the cofferdam 10 is decreased and thus the heat transfer between the LNG and the cofferdam 10 is more decreased than the related art, which leads to the decrease in the BOR.

According to the embodiment of the present invention, when the bulk head of the cofferdam is made of a material that may bear a temperature from −30° C. to 0° C., the temperature of the cofferdam may be changed in a range from −30° C. to 70° C. and when the bulk head of the cofferdam is made of low temperature steel LT that may bear up to a temperature of −55° C., in detail, a temperature from −31° C. to −55° C., the temperature of the cofferdam may be changed in a range from −55° C. to 70° C.

In detail, as illustrated in a table of FIG. 7, when the temperature of the bulk head 11 of the cofferdam 10 is controlled to be −25° C., the temperature of the cofferdam 10 may be maintained at −20.8° C. In this case, the BOR becomes 0.1236 and it may be appreciated that the value is 3.5% smaller than the BOR of the related art.

Further, as illustrated in the table of FIG. 7, when the temperature of the bulk head 11 of the cofferdam 10 is controlled to be −50° C., the temperature of the cofferdam 10 may be maintained at −46.5° C. In this case, the BOR becomes 0.1192 and it may be appreciated that the value is 7.0% smaller than the BOR of the related art. For reference, the foregoing value of the BOR is a result of numerical analysis.

However, when the temperature of the cofferdam 10 is maintained at the temperature below 0° C., the bulk head 11 needs to be made of the materials defined in the IGC or the low temperature steel LT and therefore costs may be expected to be increased. However, the increase in costs is smaller than a profit generated at the time of the decrease in the BOR, and therefore the BOR may be effectively decreased with relatively lower cost.

Further, a loss of LNG evaporated as the BOG due to the decrease in the BOR may be prevented and therefore the foregoing increase in costs may be sufficiently offset.

Describing in detail the cofferdam 10, according to the present embodiment, as illustrated in FIG. 1, the cofferdam includes a pair of bulk heads 11 disposed between the plurality of LNG storage tanks T while being spaced apart from each other and a space portion 12 provided by the pair of bulk heads 11 and an internal hull IH and may control the pair of bulk heads 11 to be temperature below 0° C. to control the temperature of the cofferdam 10 to be temperature below 0° C.

According to the present embodiment, the temperature of the cofferdam 10 may be controlled to be temperature below 0° C. by, for example, controlling a set temperature at which a heating system of the cofferdam 10 is operated, additionally installing a heat insulating material 120 (see FIG. 9) in the cofferdam 10, or injecting cooled gas in the cofferdam 10.

In detail, upon designing an LNG carrier, external air temperature becomes −18° C. depending on USCG conditions and the LNG carrier needs to be designed without any problem even when sea temperature is 0° C. Under the external temperature condition, when the cofferdam 10 is not heated, the cofferdam 10 is dropped up to −60° C. by cold heat of the LNG stored in the LNG storage tank T.

Therefore, the related art heats the cofferdam 10 to control the temperature of the space portion 12 of the cofferdam 10 to be 5° C. and the temperature of the bulk head 11 to be 0° C. or more all the time.

However, according to the present embodiment, unlike the existing LNG carrier, in the temperature below 0° C. proposed in the present embodiment, a heater may be operated to control the temperature of the cofferdam 10 to be a specific sub-zero temperature.

Further, the heat insulating material 120 (see FIG. 9) may be installed inside the cofferdam 10 to control the temperature of the cofferdam 10 to be temperature below 0° C. and the heat insulating material 120 will be described in detail in a second embodiment to be described below.

According to the present embodiment, the foregoing method for controlling the temperature of the cofferdam 10 to be temperature below 0° C. may also be independently used and may also be used like other methods, and therefore the scope of the present invention is not limited to applying any one method.

The temperature of the bulk head 11 of the cofferdam 10 is controlled to be temperature below 0° C. and therefore the bulk head 11 may be made of B, D, E, AH, DH, and EH that are a steel grade defined in the IGC.

In particular, when the temperature of the bulk head 11 of the cofferdam 10 is controlled to be a temperature from −30° C. to −20° C., the bulk head 11 may be made of E or EH that is the steel grade defined in the IGC and when the temperature of the bulk head 11 is controlled to be temperature from −60° C. to −30° C., the bulk head 11 may be made of the low temperature steel LT.

According to the present embodiment, when the bulk head 11 is made of the low temperature steel, the low temperature steel may be made of one of low temperature carbon steel, low temperature alloy steel, nickel steel, aluminum steel, and austenite-based stainless steel or at least one combination thereof.

Further, as illustrated in FIGS. 1 and 3, when the cofferdam 10 is disposed in one row in the width direction of the hull, in the space portion 12, the pair of bulk heads 11 spaced apart from each other in the length direction of the hull may form a front wall 7 a and a rear wall 9 a and the internal hull IH may form left and right side walls, a ceiling part, and a bottom part.

Further, according to the present embodiment, as illustrated in FIG. 4, the cofferdam 10 includes a lateral cofferdam 10 a laterally partitioning an internal space of the LNG storage tank T and a longitudinal cofferdam 10 b longitudinally partitioning it.

In this case, in the space portion 12 of the cofferdam 10, in the case of the lateral cofferdam 10 a, as illustrated in FIG. 4, the pair of bulk heads 11 spaced apart from each other in the length direction of the hull may each form a front wall and a rear wall of the space portion 12, the right internal hull IH may form a right wall 3 a, a left partition wall may form a left wall 5 a, and the internal hull IH may form a ceiling wall and a floor wall.

Further, in the case of the longitudinal cofferdam 10 b, as illustrated in FIG. 4, the pair of bulk heads 11 spaced apart from each other in the width direction of the hull may each form the right wall and the left wall of the space portion 12, a wall contacting the bulk head 11 of the longitudinal cofferdam 10 b and bulk head 11 of the lateral cofferdam 10 a may form a front wall 7 a and a rear wall 7 b, and the internal hull IH may form the ceiling wall and the floor wall.

According to the present embodiment, the space portion 12 may be provided with the heat insulating material 120 according to the second embodiment of the present invention to be described below, and the heat insulating material 120 will be described in detail in the second embodiment of the present invention.

A gas supplier supplies gas into the cofferdam 10 to serve to prevent the cofferdam 10 from being damaged due to frost covered on the cofferdam 10, a change in humidity, or the like.

According to the present embodiment, the gas supplier may be configured to be the same as a gas supplier 300 (see FIG. 17) according to a fourth embodiment of the present invention to be described below and includes a supply pipe branched from a gas supply line to supply gas supplied through the gas supply line into the cofferdam 10, a discharge pipe provided in the cofferdam 10 to discharge gas filled in the cofferdam 10 to the outside of the cofferdam 10, and valves provided in the supply pipe and the discharge pipe.

The supply pipe of the gas supplier may be provided in the number corresponding to the number of cofferdams 10 and a lower end portion of the supply pipe may be disposed to be close to the bottom of the cofferdam 10.

The discharge pipe of the gas supplier may be provided in the number corresponding to the number of cofferdams to discharge the gas filled in the cofferdam 10 and the discharge pipes may also discharge gas by being connected to each other.

The valve for the gas supplier 20 may be a proportional control valve that is opened and closed by an electrical signal.

According to the present embodiment, the gas supplied to the gas supply line includes dry air, inert gas, or N₂ gas and the gas may be supplied from the existing dry air/inert gas generator that is already installed in the LNG carrier.

Meanwhile, according to the present embodiment, the floating structure may include a heater 30 controlling the temperature of the cofferdam 10 dropped to a first sub-zero temperature to be a second sub-zero temperature higher than the first sub-zero temperature.

According to the present embodiment, as illustrated in FIG. 8, the heater 30 may also heat the bulk head 11 by installing a glycol heating coil 31 in the cofferdam 10 and supplying heated glycol to the glycol heating coil 31 and may also heat the bulk head 11 by installing an electrical coil in the cofferdam 10.

Further, a coil through which waste heat of exhaust gas or high temperature liquid or steam may be circulated may also be provided in the cofferdam 10 to heat the bulk head 11.

According to the present embodiment, when the glycol is used as an anti-freezing solution, 45% of glycol water having a freezing point of −30° C. may be used.

A method for heating glycol supplied to a cofferdam 10 will be briefly described with reference to FIG. 8.

The glycol circulated by a glycol circulation pump is heated with the high temperature steam supplied from a boiler, or the like in a glycol heater GH prior to being supplied to the cofferdam 10 and the heated glycol is supplied to the glycol heating coil 31 provided in the cofferdam 10 to heat the bulk head 11 and then be circulated.

According to the present embodiment, the cofferdam 10 may be provided with a temperature sensor TS that may measure the temperature of the inside of the cofferdam 10 and when the temperature of the inside of the cofferdam 10 is lower than the set value, the heated glycol may be supplied to the glycol heating coil 31 attached to the bulk head 11 to increase or maintain the temperature of the bulk head 11 and the space portion 12.

Meanwhile, since the freezing point of the anti-freezing solution may be dropped to −50° C. or less when the temperature of the bulk head 11 is controlled to be −50° C. or less, as the anti-freezing solution, 65% of glycol water or methyl alcohol may be used. The contents described in the first embodiment of the present invention may be applied to other embodiments to be described below as it is.

FIG. 9 is a diagram schematically illustrating a state in which a heat insulating material is provided in a cofferdam in a heat insulating system of a floating structure according to a second embodiment of the present invention, FIG. 10 is a perspective view schematically illustrating a state in which the heat insulating material is provided in region “A” of FIG. 9, FIG. 11 is a perspective view schematically illustrating a state in which the heat insulating material is provided in region “B” of FIG. 9, FIG. 12 is diagrams illustrating a modified example of the heat insulating material provided in region “C”, and FIG. 13 is a table illustrating calculated results of a BOR generated by controlling the temperature of the cofferdam using the heat insulating material illustrated in FIG. 9.

A heat insulating system 100 of the floating structure according to the present embodiment includes the heat insulating material 120 provided in the cofferdam 10 for controlling the temperature of the cofferdam 10 to be temperature below 0° C., independent of spatial environment such as a polar region and a tropical region or temporal environment such as season and week/day and night.

As illustrated in FIG. 9, the heat insulating material 120 is provided in the cofferdam 10 to serve to prevent heat from being invaded into the cofferdam 10 when the floating structure is sailed in a high temperature region or during the summer to drop the temperature of the cofferdam 10 to the desired temperature even when the outside temperature is high.

In detail, when the outside temperature is high, for example, when the outside air temperature proposed in an IGC code is 45° C. and the sea temperature is 25° C., as illustrated in a table of FIG. 13, without the heat insulating material 120, the temperature of the cofferdam 10 may be dropped only to −15.39° C. and thus the decrease in the BOR may be limited.

However, according to the present embodiment, if the cofferdam 10 is provided with the heat insulating material 120, the temperature of the cofferdam 10 is lowered to the desired temperature, for example, −25° C. and −50° C. even under the foregoing temperature conditions and thus the sufficient BOR decrease effect may be obtained.

Describing in detail the heat insulating material 120, according to the present embodiment, the heat insulating material 120 may use a heat insulating wall that is a type different from the foregoing type of heat insulating wall in consideration of operation convenience and cost, etc., as well as the heat insulating wall used to heat-insulate the LNG stored in the LNG storage tank T.

That is, according to the present embodiment, the heat insulating material 120 may include at least one of a panel type heat insulating material, a foam type heat insulating material, a vacuum heat insulation or particle type heat insulating material, and a non-woven fabrics type heat insulating material that are types different from the foregoing heat insulating wall.

According to the present embodiment, the heat insulating material 120 may be applied without a limitation of kind and shape. Only any one of the foregoing three types of heat insulating materials may be used in consideration of the operation environment and cost, etc., and at least two heat insulating materials may also be selectively used. In addition, the heat insulating wall heat-insulating the LNG stored in the LNG storage tank T may also be used. Here, the heat insulating wall refers to a heat insulating wall of a sealing and heat insulating unit SI.

The panel type heat insulating material includes styrofoam, in which the styrofoam may be coupled to the cofferdam 10 by an attachment scheme using a low temperature adhesive, a bolt, etc.

The foam type heat insulating material includes polyurethane foam, in which the polyurethane foam may be injected to the cofferdam 10 by a foaming scheme to be coupled to the cofferdam 10.

The non-woven fabrics type heat insulating material may also be made of a polyester fiber material or a synthetic resin and may be coupled to the cofferdam 10 by the attachment scheme using the low temperature adhesive, the bolt, etc.

According to the present embodiment, the kind and installation method of the heat insulating material 120 are not limited.

According to the present embodiment, as illustrated in FIG. 9, the heat insulating material 120 may be provided in the space portion 12 of the cofferdam 10 of a region other than the pair of bulk heads 11.

In detail, as illustrated in FIG. 10, in the case of the lateral cofferdam 10 a, the heat insulating materials 120 may be each provided at a right wall part, a left wall part, a ceiling part, and a bottom part of the space portion 12 of the cofferdam 10. Further, the heat insulating material 120 provided at the ceiling part and the bottom part may not be provided inside the space portion 12 but provided outside the space portion 12.

If the heat insulating material 120 is provided in the cofferdam 10 of the region other than the pair of bulk heads 11, heat outside the region that does not contact the pair of bulk heads 11 may be prevented from being invaded into the cofferdam 10 and the cold heat of the LNG stored in the LNG storage tank T may be transferred to the space portion 12 through the pair of bulk heads 11, such that the temperature of the cofferdam 10 may be lowered to the desired temperature even when the temperature of the outside of the hull is high.

Further, according to the embodiment of the present invention, the heat insulating materials 120 may be provided in a foremost bulk head 11 of a bow of the lateral cofferdam 10 a disposed at a foremost side of the bow and a rearmost bulk head of a stern of the lateral copper dam 10 a disposed at a rearmost side of the stern, respectively, among the plurality of lateral cofferdams 10 a.

In detail, FIG. 11 illustrates that the heat insulating material 120 is provided in the bulk head 11 of the foremost side of the bow, in which the foremost side of the bow and the rearmost side of the stern have environment different from a region between the bow and the stern.

That is, the regions of the foremost side of the bow and the rearmost side of the stern contact the LNG storage tank T only in one direction and contact the inner wall of the hull, such that it is more difficult to lower the temperature of the cofferdam 10 to the desired temperature than lowering the temperature of the cofferdam 10 disposed in the region between the bow and the stern.

However, according to the present embodiment, if the foremost bulk head 11 of the bow and the rearmost bulk head 11 of the stern are provided with the heat insulating material 120, heat may be prevented from being invaded from the outside and therefore the temperature of the cofferdam 10 may be lowered to the desired temperature.

Meanwhile, when the inside of the cofferdam 10 is provided with the heat insulating material 120, the heat insulating material 120 provided at the bottom part of the cofferdam 10 may be damaged due to a crew That is, when a worker enters the cofferdam 10, he/she stands on the bottom part of the cofferdam 10 with his/her feet. In this case, the heat insulating material 120 may be damaged.

Therefore, according to the embodiment of the present invention, to prevent the heat insulating material 120 from being damaged as described above, as illustrated in FIG. 12, a heat insulating material damage prevention member may be provided.

According to the embodiment of the present invention, as illustrated in FIG. 12A, a heat insulating material damage prevention member 130 a may be provided in a grid form and disposed on the heat insulating material 120 to prevent a load from being concentrated on a specific part of the heat insulating material 120, thereby preventing the heat insulating material 120 from being damaged.

Further, a heat insulating material damage prevention member 130 b may be a separate path provided at the bottom part of the cofferdam 10 to move a crew to a desired place. A main area to which a crew is approached is an edge of the bottom part, and therefore as illustrated in FIG. 12B, the heat insulating material damage prevention member 130 b may be provided only at the edge of the bottom part of the cofferdam 10, having a slight width.

FIG. 13 illustrates the decrease effect of the BOR by the installation of the heat insulating material and the temperature control of the cofferdam.

As before, if the cofferdam is controlled to be 5° C., the BOR becomes about 0.1282. Here, to control the temperature of the cofferdam, even when the control temperature of the glycol heating system is controlled, that is, when the glycol heating is not performed, the cofferdam may be dropped only to −10.87° C. even at the time of the lowest temperature.

Therefore, even though the bulk head 11 of the cofferdam is made of the steel grade E that may bear up to −25° C., the temperature of the cofferdam is dropped only to −15.39° C. and therefore the BOR is decreased only by about 2.2%.

However, by applying the present embodiment, if the heat insulating material 120 is installed to drop the temperature of the cofferdam 10 up to −26.4° C. and increase the temperature of the cofferdam 10 up to −20.8° C. by the glycol heating, the BOR may be decreased by about 3.5.

Further, the contents of the first embodiment of the present invention as described above may be applied to the present embodiment as it is.

FIG. 14 is a diagram schematically illustrating a state in which a bulk head of a cofferdam in a floating structure according to a third embodiment of the present invention is not extended to an external hull but is connected only to an internal hull, FIG. 15 is a modified example of FIG. 14 in which instead of the bulk head illustrated in FIG. 14, the cofferdam is provided and the heat insulating material is provided in the cofferdam, and FIG. 16 is a table illustrating calculated results of a BOR generated by manufacturing the bulk head illustrated in FIG. 13 using a very low temperature material and controlling the temperature of the cofferdam.

A heat insulating system 200 of the floating structure according to the present embodiment includes a bulk head 210 that is provided between the plurality of LNG storage tanks T to dispose the plurality of LNG storage tanks T in multi rows in at least one of a length direction and a width direction of the hull and is not extended up to an external hull EH but is connected only to the internal hull IH, a strength member 220 that connects between the internal hull IH and the external hull EH to reinforce the internal hull IH and the external hull EH and is not continued from the bulk head 210, the heat insulating material 120 provided at the foremost side of the bow and the rearmost side of the stern, the gas supplier 20 that supplies gas to the space portion 12 provided by the bulk heads 210 at the foremost side of the bow and the rearmost side of the stern to prevent the space from being damaged due to the change in humidity, and the heater 30 that heats the bulk heads 210 provided at the foremost side of the bow and the rearmost side of the stern.

As illustrated in FIG. 14, the bulk head 210 may dispose the LNG storage tank T in the multi row in the length direction of the hull and in the multi row in the width direction of the hull.

Further, according to the present embodiment, the region to which the bulk head 210 and the LNG storage tank T are contacted is not provided with the sealing and heat insulating unit SI and the heat insulating material 120 and therefore the temperature of the bulk head 210 may be dropped to a very low temperature of −140° C.

Therefore, according to the present embodiment, the bulk head 210 may be made of the very low temperature materials including stainless steel or aluminum and an end portion of a sealing wall of the sealing and heat insulating unit SI sealing and heat-insulating the LNG storage tank T may be directly welded to the bulk head 210.

Further, according to the present embodiment, a pair of bulk heads 210 may be spaced apart from each other at the foremost side of the bow and the rearmost side of the stern to provide the space portions 12 at the foremost side of the bow and the rearmost side of the stern. The bulk head 210 of the space portion 12 may be provided with the heat insulating material 120 and the heater 30 and the gas supplier may supply gas to the space portion 12 to prevent the bulk head 210 from being damaged.

Meanwhile, unlike the related art, according to the present embodiment, as illustrated in FIG. 14, the bulk head 210 is not extended up to the external hull EH. The reason is that if the bulk head 210 is connected up to the external hull EH, heat is transferred from the outside through the bulk head 210 and thus the BOR may also be increased and the external hull EH contacts the bulk head 210 and therefore the brittle fracture may occur due to the cold heat transferred from the bulk head 210.

As illustrated in FIG. 14, the strength member 220 connects between the internal hull IH and the external hull EH at an intermediate position of the LNG storage tank T to serve to structurally reinforce the hull.

As illustrated in FIG. 14, according to the present embodiment, the strength member 220 is provided not to be continued from the bulk head 210 and therefore the cold heat transferred through the bulk head 210 may be offset by the sealing and heat insulating units SI provided at both end portions of the bulk head 210 and the bulk head 210 does not directly contact the external hull EH and therefore the heat transfer from the outside may also be decreased.

According to the present embodiment, the strength member 220 may be provided even at any position as long as it is disposed not to be continued from the bulk head 210 and the number of strength members 220 is not limited.

Further, the strength member 220 is not exposed to very low temperature and therefore may also be made of steel that is the steel grade A.

As the heat insulating material 120, the heat insulating material 120 according to the foregoing second embodiment may be applied as it is. However, there is a difference in that the heat insulating material 120 is not provided between the LNG storage tanks T but is provided at the foremost side of the bow and the rearmost side of the stern, in the installation position.

The gas supplier and the heater 30 may be applied like the foregoing first embodiment. However, there is a difference from the foregoing first embodiment in that they are applied to the space portions 12 provided at the foremost side of the bow and the rearmost side of the stern.

According to the present embodiment, not the cofferdam 10, the bulk head 210 is provided between the LNG storage tanks T and thus it is difficult to directly control the temperature of the bulk head 210 disposed between the LNG storage tanks T, such that the foregoing bulk head 210 is controlled to be about −130° C. due to the direct contact with LNG.

However, the temperature of the bulk heads 210 disposed at the foremost side of the bow and the rearmost side of the stern may be freely controlled by the heater 30 and even in the case of the bulk head 210 disposed between the LNG storage tanks T, the temperature of the bulk head 210 may be controlled by controlling the heat insulating wall of the sealing and heat insulating unit SI or heating both ends of the bulk head 210 with an electrical coil.

Further, as illustrated in FIG. 15, the bulk head 210 may also be provided in at least two, the bulk heads 210 provided in at least two may also be spaced apart from each other, and the present embodiment may also be applied to a double hull structure configured of the internal hull IH and the external hull EH.

Meanwhile, according to the present embodiment, as illustrated in FIGS. 14 and 15, the region to which the bulk head 11 and the LNG storage tank T are contacted may not be provided with the sealing and heat insulating unit SI. In this case, if the bulk head 11 is made of the very low temperature material and the temperature of the cofferdam 10 is controlled to be temperature below 0° C., the BOR may be obtained as illustrated in FIG. 16.

In detail, as illustrated in FIG. 14, if the region to which the bulk head 11 and the LNG storage tank T are contacted is not provided with the sealing and heat insulating unit SI, the cold heat of the LNG stored in the LNG storage tank T is transferred to the cofferdam 10 well and thus the temperature of the cofferdam 10 may be dropped to −125° C. as illustrated in FIG. 16. In this case, it may be appreciated that the BOR is 0.1061 decreased by 17.2%, compared to the case of controlling the temperature of the cofferdam 10 to be 5° C.

In this case, the bulk head 11 of the cofferdam 10 may be controlled to be a temperature from −163 to −50° C., the bulk head 11 may be made of the very low temperature material including stainless steel or aluminum, not a general material, and the sealing wall of the sealing and heat insulating unit SI contacting the bulk head 11 may be coupled to the bulk head 11 by a welding scheme.

FIG. 17 is a diagram schematically illustrating a gas supplier in a floating structure according to a fourth embodiment of the present invention and FIG. 18 is a table illustrating calculated results of a BOR generated by controlling a temperature of a cofferdam illustrated in FIG. 17.

A floating structure 300 according to an embodiment of the present invention includes the cofferdam 10 provided between the plurality of LNG storage tanks T to dispose the plurality of LNG storage tanks T in multi rows in at least any one direction of the length direction and the width direction of the hull and controlled to be temperature below 0° C., a gas supplier 320 supplying gas to the cofferdam 10, the heater 30 provided in the cofferdam 10 to heat the cofferdam 10 so as to allow a worker to enter the internal space of the cofferdam 10, and the heat insulating material 120 provided in the cofferdam 10.

The present embodiment has a difference from the foregoing first and second embodiments in that the floating structure includes the gas supplier 320 supplying gas into the cofferdam 10 to easily find out a cold spot formed in the bulk head 11 of the cofferdam 10. The cofferdam 10, the heater 30, and the heat insulating material 120 described in the foregoing first and second embodiments may be applied to the present embodiment as they are.

In the floating structure according to the present embodiment, a worker also periodically enters the cofferdam 10 to inspect whether there is a cold spot in the bulk head 11 of the cofferdam 10. That is, it needs to be inspected whether a cold part occurs at a specific part of the bulk head 11 of the cofferdam 10. From this, it may be appreciated that the bulk head 11 is covered with frost and a visual inspection is performed.

However, when the temperature of the cofferdam 10 is maintained to be lower at temperature below 0° C. and the inside of the cofferdam 10 is filled with general air, the whole bulk head 11 of the cofferdam 10 is covered with frost and therefore the cold spot may not be found based on presence and absence of the frost.

According to the present embodiment, the cofferdam 10 is filled with gas, for example, the dry air and the temperature of the bulk head 11 of the cofferdam 10 is controlled to be higher than a dew point of the dry air to cover only the bulk head 11 having temperature lower than the dew point temperature of the dry air with frost, thereby easily finding out the cold spot.

For example, when the dew point temperature of the dry air generated in the LNG carrier is −40° C., the temperature of the bulk head 11 of the cofferdam 10 is controlled to be −35° C. and if a worker enters the cofferdam 10 to perform a visual inspection, the bulk head 11 of the cofferdam 10 having a temperature lower than −40° C. is covered with frost, and therefore the cold spot may be easily found based on the position of the frost.

Further, a technology means for supplying the dry air having the lower dew point temperature into the foregoing cofferdam 10 may be applied to a trunk deck space TS (see FIG. 21) and a side passage way SP (see FIG. 21) contacting a trunk deck TD as it is in a sixth embodiment of the present invention to be descried below.

Further, when the temperature of the bulk head 11 of the cofferdam 10 is controlled to be −35° C., as shown in a Table of FIG. 18, the BOR may be decreased by about 4.9% compared to the case of controlling the temperature of the bulk head 11 to be 5° C. In this case, the bulk head 11 may be made of the low temperature steel LT.

According to the present embodiment, as illustrated in FIG. 17, the gas supplier 320 includes a gas supply pipe 321 provided in the cofferdam 10 to supply gas supplied through a gas supply line AL into the cofferdam 10, a gas discharge pipe 322 provided in the cofferdam 10 to discharge the internal gas of the cofferdam 10 to the outside of the cofferdam 10, and shutoff valves 323 provided in the gas supply pipe 321 and the gas discharge pipe 322.

According to the present embodiment, the dry air supplied to the gas supply line AL may be supplied from a dry air generator installed in the existing LNG carrier and therefore additional costs for the facility do not occur.

According to the present embodiment, the dry air supplied to the cofferdam 10 may have the dew point temperature from −45° C. to −35° C. and the temperature of the bulk head 11 of the cofferdam 10 may be controlled to be 1° C. to −10° C. higher than the dew point temperature of the dry air. In this case, the temperature of the bulk head 11 is controlled to be about −30° C., and therefore the BOR may be decreased.

For the reason of inspection, maintenance, etc., when a worker needs to enter the cofferdam 10, he/she may put on winter clothes to bear low temperature, thereby performing the work. On one hand, the foregoing gas is continuously injected into and vented from the cofferdam 10 to increase the temperature of the cofferdam, and as a result, a worker may enter the cofferdam to perform the work.

FIG. 19 is a diagram schematically illustrating controlling the temperature of the cofferdam depending on a change in pressure of an LNG storage tank in a floating structure according to a fifth embodiment of the present invention.

A heat insulating system 400 of the floating structure according to the present embodiment includes the cofferdam 10 that is provided between the plurality of LNG storage tanks T to dispose the plurality of LNG storage tanks T in multi rows in at least one direction of the length direction and the width direction of the hull and is controlled to be temperature below 0° C. and the heater 30 that is provided in the cofferdam 10 to heat the cofferdam 10, in which the present embodiment has a difference from the foregoing first embodiment in that the cofferdam 10 is heated by the heater 30 to control the sub-zero temperature of the cofferdam 10 to be temperature above 0° C. and the temperature of the cofferdam 10 is controlled depending on the change in pressure in the LNG storage tank T and the rest contents of the first embodiment may be applied to the present embodiment as they are.

That is, according to the present embodiment, when the temperature of the cofferdam 10 is maintained at temperature below 0° C. to lower the BOR and the BOG is little generated according to the sailing conditions, and thus more BOG is required for the reason of ship fuel, etc., the temperature of the cofferdam 10 is increased to make the BOR larger and generate more BOG and when too much BOG is generated according to the sailing conditions and thus it is difficult to treat the BOG, the temperature of the cofferdam 10 is lowered to make the BOR smaller and less generate the BOG.

The foregoing control temperature may be manually set in consideration of the sailing conditions, etc., and may also be automatically set by receiving a pressure signal of the LNG storage tank T. That is, when the pressure of the LNG storage tank T is high, the BOG is excessively generated and therefore the set value of the control temperature may be controlled to be low and when the pressure is low, the BOG is less generated and therefore the set value of the control temperature may be controlled to be high.

Further, the present embodiment has a difference from the foregoing first embodiment in that the temperature of the cofferdam 10 is maintained at temperature below 0° C. to decrease the BOR and the temperature of the cofferdam 10 may be controlled to be a specific temperature (for example, temperature above 0° C.) so that a worker may enter the cofferdam 10.

In detail, to inspect whether the cold spot, etc., occurs in the cofferdam 10 during the sailing, a worker needs not enter the cofferdam 10.

Even in this case, if the cofferdam 10 is maintained at temperature below 0° C., a worker entering the cofferdam 10 to perform the work is exposed to a low temperature and thus may be in danger. Therefore, the set value of the control temperature is increased and thus the heater 30 heats the cofferdam 10 to maintain the cofferdam 10 at the specific temperature (for example, temperature above 0° C.)

According to the present embodiment, when the bulk head 11 of the cofferdam 10 is made of a material bearing a temperature from −30° C. to 0° C., the temperature of the cofferdam 10 may be controlled to be in a range from −30° C. to 70° C. For example, when a worker need not to enter the cofferdam 10, to maximally decrease the BOR, the control temperature of the cofferdam 10 may be controlled to be about −30° C. On the other hand, the cofferdam 10 may be controlled to be a specific temperature in addition to temperature above 0° C.

According to the present embodiment, when the bulk head 11 of the cofferdam 10 is made of the low temperature steel LT bearing up to a temperature of −55° C., the temperature of the cofferdam 10 may be controlled to be in a range from −55° C. to 70° C. For example, when a worker need not to enter the cofferdam 10, to maximally decrease the BOR, the temperature of the cofferdam 10 may be controlled to be about −50° C. On the contrary, the cofferdam 10 may be controlled to be a specific temperature in addition to temperature above 0° C.

Hereinafter, to permit a worker to enter the cofferdam 10, a method for controlling a temperature of a cofferdam will be described.

First, the temperature of the cofferdam 10 is controlled to temperature below 0° C., for example, −25° C. or −50° C. to decrease the heat transfer between the cofferdam 10 and the LNG stored in the LNG storage tank T, and therefore if a worker immediately enters the cofferdam, he/she may be in danger.

Therefore, a step of heating the cofferdam 10 at temperature above 0° C. by the heater 30 is performed. In this case, the cofferdam 10 may be heated with a glycol heating coil 31, an electrical coil, and a coil in which steam or clear water flows or heated by supplying high temperature air into the cofferdam 30.

Next, if the temperature in the cofferdam 10 becomes temperature above 0° C., a worker enters the cofferdam 10 to confirm whether the cold spot, etc., occurs in the bulk head 11. In this case, the inside of the cofferdam 10 is continuously maintained at temperature above 0° C.

If a worker completes the internal inspection of the cofferdam 10 to get out of the cofferdam 10, the heating of the cofferdam 10 stops to again maintain the cofferdam 10 at temperature below 0° C.

As described above, according to the present embodiment, when a worker does not enter the cofferdam 10, the cofferdam 10 may be maintained at temperature below 0° C. to decrease the BOR. On the other hand, the cofferdam 10 may be maintained at temperature above 0° C. to permit a worker to perform the work and consider safety of a worker while decreasing the BOR.

Further, the foregoing technology means for controlling a temperature of a cofferdam 10 may also be applied to the trunk deck space TS (see FIG. 21) and the side passage way SP (see FIG. 21) contacting the trunk deck TD as it is in the sixth embodiment of the present invention to be described above.

Further, the present embodiment has a difference from the foregoing first embodiment in that the temperature of the cofferdam 10 may be controlled depending on the change in pressure in the LNG storage tank T.

In detail, according to the present embodiment, as illustrated in FIG. 19, a pressure sensor PT that may measure the pressure in the LNG storage tank T is provided in the LNG storage tank T and then the temperature of the cofferdam 10 may be controlled based on the pressure measured by the pressure sensor PT.

That is, if the pressure of the LNG storage tank T is increased, the BOG is more generated than the BOG required in the floating structure and therefore the setting temperature of the temperature control of the cofferdam 10 is lowered to lower the temperature of the cofferdam 10, thereby decreasing the BOG. If the pressure of the LNG storage tank T is decreased, the BOG is less generated than the BOG required in the floating structure and therefore the setting temperature of the temperature control of the cofferdam 10 is increased to increase the temperature of the cofferdam 10, thereby generating more BOG.

Further, the temperature of the cofferdam 10 may also be controlled by referring to a velocity of the floating structure independent of the pressure sensor PT.

In detail, when the velocity of the floating structure is increased to increase the fuel consumption, the control temperature of the cofferdam 10 may be increased to more generate BOG and the generated BOG may be used as fuel to fit the fuel consumption.

For example, in the floating structure controlling the bulk head 11 of the cofferdam 10 to be the setting temperature of −25° C., the BOR becomes 0.1236. When the velocity of the floating structure is increased to consume more fuel, if the temperature of the bulk head 11 of the cofferdam 10 is controlled to be 0° C., the BOR becomes 0.1282 and thus is increased by 3.7%, thereby increasing the BOG. Therefore, when the velocity of the floating structure is increased and thus the consumption of the BOG is increased, the insufficient amount of BOG may be decreased.

On the contrary, when the velocity of the floating structure is decreased and thus the fuel consumption is decreased, the control temperature of the cofferdam 10 is lowered to less generate the BOG, thereby fitting the fuel consumption.

Meanwhile, when the bulk head 11 of the cofferdam 10 is heated by the heater 30, the heat transfer is made by conduction and thus the heating time of the cofferdam 10 may be required, such that the hot dry air may be supplied into the cofferdam 10 to shorten the heating time of the cofferdam 10.

In addition, the gas supplier and the gas supplier 320 described in the foregoing embodiments may be applied to the present embodiment as they are.

FIG. 20 is a diagram schematically illustrating a state in which a heat insulating material is provided in a trunk deck space TS and a side passage way in a heat insulating system of a floating structure according to a sixth embodiment of the present invention and FIG. 21 is a table showing calculated results of a BOR generated by controlling a temperature of an internal hull IH contacting the trunk deck space TS and the side passage way illustrated in FIG. 20.

A heat insulating system 500 of the floating structure according to the present embodiment includes the heat insulating material 120 provided in at least one of the trunk deck space TS and the side passage way SP contacting the trunk deck TD to decrease the heat transfer from the trunk deck space TS or the side passage way SP into the plurality of LNG storage tank T, thereby decreasing the BOR generated due to the heat transfer.

According to the present embodiment, the temperature of the internal hull IH contacting the trunk deck space TS and the side passage way SP may be lowered to decrease the heat invasion from the outside, thereby decreasing the BOR.

In particular, in the case of sailing the floating structure along a route having very low ambient temperature like a north pole route or sailing the floating structure during the winter, if the present embodiment is applied, the temperature of the internal hull IH contacting the trunk deck space TS and the side passage way SP may be lowered to decrease the BOR.

On the contrary, even in the case of sailing the floating structure at a place where temperature is high or sailing the floating structure during the summer, the temperature of the internal hull IH contacting the trunk deck space TS and the side passage way SP is lowered by the heat insulating material 120 to maintain the temperature of the cofferdam 10 at low temperature, thereby decreasing the BOR.

In particular, the trunk deck TD and the side passage way SP contacting the trunk deck TD are directly exposed to external solar heat and therefore if the heat insulating material 210 is provided here, the heat invasion from the outside may be decreased and thus the BOR may be more effectively decreased.

As a result of calculating the BOR for the actual LNG carrier based on numerical analysis, as illustrated in a table in FIG. 21, when the temperature of the internal hull IH contacting the trunk deck space TS and the side passage way SP is not controlled, the temperature of the internal hull IH becomes about 35.3° C. In this case, the BOR is calculated as 0.1346.

However, when the present embodiment is applied to control the temperature of the internal hull IH contacting the trunk deck space TS and the side passage way SP to be 0° C., as shown in a table of FIG. 21, it may be appreciated that the BOR is decreased by about 3.7% as 0.1296. The BOR may be decreased using the cheap heat insulating material 120 and it may be appreciated that the great BOR decrease effect compared to price may be obtained.

As another example, when the present embodiment is applied to control the temperature of the internal hull IH contacting the trunk deck space TS and the side passage way SP to be −25° C., it may be appreciated that the BOR becomes 0.1266 and thus is decreased by about 5.9%. Likewise, it may be appreciated that upon the use of the cheap heat insulating material 120, the great BOR decrease effect compared to price may be obtained.

As illustrated in FIG. 20, the heat insulating material 120 may be provided at an inside ceiling part of the trunk deck TD, a ceiling part and a side wall part of the side passage way SP contacting the trunk deck TD, and a part of the side passage way SP contacting a ballast tank BT.

According to the present embodiment, the heat insulating material 20 is not provided only at the position of the foregoing trunk deck TD but may be provided at the bottom part or the outside part of the trunk deck TD and may also be discontinuously or continuously provided in the trunk deck space TS and the side passage way SP.

Further, the present embodiment may use the heat insulating material 120 of the foregoing embodiment as it is. That is, the heat insulating material 120 of the present embodiment may be a heat insulating wall of the sealing and heat insulating unit SI sealing and heat insulating the LNG storage tank T and may include at least one of the panel type heat insulating material, the foam type heat insulating material, the vacuum heat insulation or particle type heat insulating material, and the non-woven fabrics type heat insulating material that are different from the type of the heat insulating material. Further, the present invention does not limit a kind, shape, and installation method of the heat insulating material.

The present embodiment may include the heater 30 heating the internal hull IH to heat the cofferdam 10 or heating the internal hull IH to maintain the internal hull IH as the desired temperature. The configuration of the heater 30 may include the glycol heating coil 31, the electrical coil, the coil in which the steam and a liquid such as the clear water flows, or the like of the foregoing embodiment.

According to the present embodiment, the control of the material and the temperature of the internal hull IH contacting the trunk deck space TS and the side passage way SP may be selectively made depending on the required value of the BOR.

In detail, according to the present embodiment, the internal hull IH may be controlled to be −55° C. to 30° C. Preferably, to use the material of the internal hull IH as the steel grade A defined in the IGC, the internal hull IH may be controlled to be 0° C. to 30° C. For example, if the temperature of the internal hull IH is controlled to be 0° C., as shown in a table of FIG. 21, compared to the existing embodiment controlling the temperature of the internal hull IH to be 35.3° C., the BOR may be 0.1296 decreased by 3.7% and the internal hull IH may also use the steel grade A.

Further, if the temperature of the internal hull IH is controlled to be −25° C., as shown in a table of FIG. 21, the BOR may be 0.1266 decreased by 5.9% and the internal hull IH may also use the steel grade E or EH. In addition, if the temperature of the internal hull IH is controlled to be −30° C. or less, the internal hull IH may be made of the low temperature steel LT.

Meanwhile, the contents of the cofferdam 10, the gas supplier 320, and the gas supplier of the foregoing embodiment may be applied to the present embodiment as they are.

FIG. 22 is a diagram schematically illustrating a state in which a heat insulating material is provided in a ballast tank in a heat insulating system of a floating structure according to a seventh embodiment of the present invention and FIG. 23 is a table illustrating calculated results of a BOR generated by controlling a temperature of an internal hull IH contacting the ballast tank.

A heat insulating system 600 of the floating structure according to the present embodiment includes the heat insulating material 120 provided in the ballast tank BT to decrease the heat transfer from the ballast tank BT into the LNG storage tank T, thereby decreasing the BOR.

According to the present embodiment, the temperature of the internal hull IH contacting the LNG storage tank T in the ballast tank BT may be lowered to decrease the heat invasion from the outside, thereby decreasing the BOR.

Even when the floating structure is sailed toward a place where temperature is high or sailed during the summer, the temperature of the internal hull IH contacting the LNG storage tank T in the ballast tank BT may be lowered by the heat insulating material 120 to decrease the BOR.

In detail, according to the present embodiment, the temperature of the internal hull IH contacting the LNG storage tank the ballast tank BT may be controlled to be −55° C. to 30° C. Preferably, to use the material of the internal hull IH as the steel grade A defined in the IGC, the internal hull IH may be controlled to be a temperature of 0° C. to 20° C.

As a result of calculating the BOR for the actual LNG carrier based on numerical analysis, when the temperature of the internal hull IH contacting the LNG storage tank T in the ballast tank BT is not controlled, as shown in a table of FIG. 23, the temperature of the portion is about 27.2 to 36.13° C. In this case, the BOR is calculated as 0.1346.

However, when the present embodiment is applied to control the temperature of the internal hull IH contacting the LNG storage tank T in the ballast tank BT to be 0° C., as shown in a table of FIG. 23, it may be confirmed that the BOR is decreased by about 7.7% as 0.1242. That is, the BOR may be decreased using the cheap heat insulating material 120, and therefore it may be appreciated that the great BOR decrease effect compared to price may be obtained.

Further, as another example, even when the temperature of the internal hull IH contacting the LNG storage tank IN the ballast tank BT is controlled to be 5° C., it may be confirmed that the BOR is decreased by about 6.2% as 0.1262. Likewise, it may be appreciated that upon the use of the cheap heat insulating material 120, the great BOR decrease effect compared to price may be obtained.

As illustrated in FIG. 22, the heat insulating material 120 may be provide at a ceiling wall of the ballast tank BT of the region in which the inside of the external hull EH contacts the ballast tank BT and the side passage way.

Further, the present embodiment may use the heat insulating material 120 of the foregoing embodiment as it is. That is, the heat insulating material 120 of the present embodiment may be a heat insulating wall of the sealing and heat insulating unit SI sealing and heat insulating the LNG storage tank T and may include at least one of the panel type heat insulating material, the foam type heat insulating material, the vacuum heat insulation or particle type heat insulating material, and the non-woven fabrics type heat insulating material. Further, the present invention does not limit a kind, shape, and installation method of the heat insulating material.

The present embodiment may include the heater 30 heating the internal hull IH to heat the cofferdam 10 or heating the internal hull IH to maintain the internal hull IH contacting the ballast tank BT at the desired temperature. The configuration of the heater 30 may include the glycol heating coil 31, the electrical coil, the fluid coil in which the steam and the clear water flows, or the like of the foregoing embodiment.

According to the present embodiment, the control of the material and the temperature of the internal hull IH contacting the ballast tank BT may be selectively made depending on the required value of the BOR.

In detail, according to the present embodiment, the internal hull IH contacting the ballast tank BT may be controlled to be a temperature of −55° C. to 30° C. If the temperature of the internal hull IH is controlled to be 0° C., as shown in the table of FIG. 23, compared to the existing embodiment controlling the temperature of the internal hull IH to be 27.1° C. to 36.1° C., the BOR may be 0.1242 decreased by 7.7% and the internal hull IH may also use the steel grade A.

Further, if the temperature of the internal hull IH is controlled to be 5° C., as shown in the table of FIG. 23, the BOR may be 0.1262 decreased by 6.2% and the internal hull IH may use the steel grade A.

Meanwhile, the contents of the cofferdam 10 and the gas supplier 320 of the foregoing embodiment may be applied to the present embodiment as they are. However, the gas supplier 320 may not be applied in the state in which the ballast tank BT is filled with ballast water and therefore may be applied only to the cofferdam 10.

Although the embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Accordingly, such modifications, additions and substitutions should also be understood to fall within the scope of the present invention. 

1. A floating structure, comprising: a plurality of LNG storage tanks arranged in at least one row in a longitudinal direction or in a transverse direction of a hull; and a cofferdam disposed between two immediately neighboring LNG storage tanks among the plurality of LNG storage tanks or disposed on a side of one of the plurality of LNG storage tanks; wherein a temperature of the cofferdam is controlled to be temperature below 0° C. to decrease a boil-off rate (BOR) generated by transferring heat from the cofferdam into the plurality of LNG storage tanks.
 2. The floating structure of claim 1, wherein the cofferdam includes: a pair of bulk heads spaced apart from each other between the plurality of LNG storage tanks; and a space portion provided by the pair of bulk heads and an inner wall of the hull, and the cofferdam controls the pair of bulk heads to be temperature below 0° C.
 3. The floating structure of claim 2, wherein the pair of bulk heads is made of at least one material of B, D, E, AH, DH, and EH that are a steel grade defined in IGC.
 4. The floating structure of claim 1, further comprising: a gas supplier supplying gas into the cofferdam to prevent an inside of the cofferdam from being damaged due to freezing of moisture in air.
 5. The floating structure of claim 4, wherein the gas supplier includes: a supply pipe provided in the hull to supply the gas into the cofferdam; a discharge pipe provided in the cofferdam to discharge the gas in the cofferdam to an outside of the cofferdam; and valves provided in the supply pipe and the discharge pipe.
 6. The floating structure of claim 4, wherein the gas includes dry air, inert gas, or N2 gas.
 7. The floating structure of claim 1, further comprising: a heater provided in the cofferdam to heat the cofferdam, wherein the cofferdam is controlled to be temperature below 0° C. to decrease the boil-off rate (BOR) generated by transferring heat from the cofferdam into the plurality of LNG storage tanks and is heated by the heater to change the temperature below 0° C. to a specific temperature in addition to temperature above 0° C.
 8. The floating structure of claim 7, wherein when a bulk head of the cofferdam is made of a material bearing a temperature from −30° C. to 0° C., the temperature of the cofferdam is changed in a range from −30° C. to 70° C.
 9. The floating structure of claim 7, wherein when a bulk head of the cofferdam is made of low temperature steel bearing up to −55° C., the temperature of the cofferdam is changed in a range from −55° C. to 70° C.
 10. The floating structure of claim 7, wherein when fuel consumption of the floating structure is increased, the temperature of the cofferdam is increased to increase the generation of the boil-off gas (BOG) and thus the BOG is used as fuel, and when the fuel consumption of the floating structure is decreased, the temperature of the cofferdam is lowered to decrease the generation of the BOG.
 11. The floating structure of claim 7, wherein when a pressure in the LNG storage tank is larger than a set pressure of the LNG storage tank, a set temperature of the cofferdam is lowered and when the pressure in the LNG storage tank is lower than the set pressure of the LNG storage tank, the set temperature of the cofferdam is increased.
 12. The floating structure of claim 7, wherein the heater heats at least one of a trunk deck space controlled to be temperature below 0° C. and a side passage way contacting a trunk deck to change the temperature of the trunk deck space and the side passage way to the specific temperature in addition to the temperature above 0° C.
 13. The floating structure of claim 1, further comprising: a heat insulating material provided in the cofferdam.
 14. The floating structure of claim 13, wherein the cofferdam includes a plurality of lateral cofferdams that segment the plurality of LNG storage tanks laterally, and the heat insulating material is provided in a foremost bulk head of a bow of the lateral cofferdam disposed at a foremost side of the bow and a rearmost bulk head of a stem of the lateral cofferdam disposed at a rearmost side of the stem, respectively, among the plurality of lateral cofferdams.
 15. The floating structure of claim 1, further comprising: a gas supplier supplying gas to the cofferdam.
 16. The floating structure of claim 15, wherein the gas supplier includes: a gas supply pipe provided in the cofferdam to supply the gas supplied through a gas supply line into the cofferdam; a gas discharge pipe provided in the cofferdam to discharge the gas in the cofferdam to an outside of the cofferdam; and shutoff valves provided in the gas supply pipe and the gas discharge pipe.
 17. The floating structure of claim 15, wherein the gas supplied into the cofferdam has a dew point temperature from −45° C. to −35° C. and a pair of bulk heads is controlled to be 1° C. to 10° C. higher than the dew point temperature of the gas.
 18. The floating structure of claim 15, wherein the temperature of the cofferdam is maintained at temperature above 0° C. while continuously injecting and venting the gas into the cofferdam and the gas has temperature above 0° C.
 19. The floating structure of claim 15, wherein the temperature of the cofferdam is increased by continuously injecting and discharging the gas into and from the cofferdam to provide an environment that a worker enters the cofferdam.
 20. The floating structure of claim 2, wherein the bulk head is not extended up to an external hull but is connected only to an internal hull, and a strength member connecting between the external hull and the internal hull is provided not to be continued to the bulk head to decrease the boil-off rate (BOR) generated by transferring heat between the bulk head and LNG stored in the plurality of LNG storage tanks.
 21. The floating structure of claim 20, wherein the bulk head is controlled to be a temperature from −163° C. to −50° C. and is made of a very low temperature material including aluminum or stainless steel.
 22. The floating structure of claim 20, further comprising: a sealing and heat insulating unit provided in the plurality of LNG storage tanks to seal and heat-insulate the LNG, wherein the sealing and heat insulating unit is not provided in the bulk head of a region in which the plurality of LNG storage tanks and the bulk head contact each other.
 23. The floating structure of claim 22, wherein a space portion is provided between the bulk heads disposed at a foremost side of a bow and a rearmost side of a stern and the internal hull and is provided with a heat insulating material.
 24. A temperature control method of a floating structure, comprising: controlling a cofferdam at a specific sub-zero temperature to decrease a BOR; controlling the temperature of the cofferdam to a specific temperature in addition to temperature above 0° C. so that a worker enters the cofferdam controlled to be the sub-zero temperature; and controlling the temperature of the cofferdam to be the specific sub-zero temperature again when the worker gets out of the cofferdam.
 25. The temperature control method of claim 24, wherein the temperature of the cofferdam is controlled to be a range from −55° C. to 70° C. 